Table 1.

Summary of the Interpretations of Key Parameters in Dynamic Modeling

Parameter	Interpretation	Units	Example in terms of the Craving Generation Process
Gain, K	Magnitude of change in an output variable per unit change of an input variable	Output variable units/ input variable units	K 1, Craving generation process gain: the net change in current craving due to a one cigarette increase in CPD
Time constant, τ	The speed at which an output variable changes in response to a change in an input variable	Time (e.g., days)	τ1, Craving generation process time constant: reflects the speed at which current craving changes in response to a change in CPD
System zero, τa	A negative value indicates inverse response, which refers to an output variable whose initial change is in a direction opposite that of the net change	None	τa, Craving generation process’ system zero: a negative τa value indicates inverse response in current craving, that is, a change in CPD leads to an immediate increase in current craving, which subsequently settles to new, lower level